Co-led by the University of North Carolina Lineberger Comprehensive Cancer Center, the new large-scale analysis looked at data from from 15 studies carried out around the world and including a total of 889,944 women.
The researchers investigated whether there was an elevated breast cancer risk after childbirth in women younger than 55, as well as taking into account other factors such as whether the women breastfed and a family history of breast cancer.
The results, published in the Annals of Internal Medicine, showed that the risk of breast cancer was actually higher around five years after giving birth, and 80 per cent higher compared with women who did not give birth.
The finding seems to go against the common belief that childbirth has a protective effect against breast cancer, however the researchers point out that although having a child does seem to protect women against the disease, it can take more than 20 years for these benefits to occur, with the results also showing that it wasn't until 23 years after giving birth that the risk started to level off.
The risk was also not the same for all women in this age group, with results showing that the increased risk after childbirth was higher for women who also had a family history of breast cancer, or who had given birth to more children, or who had their first child after 35. There was no increased risk of breast cancer after childbirth for women who had their first child before 25.
The findings also held true whether the women breastfed or not.
The overall breast cancer risk is still low
The researchers did point out however that although breast cancer risk increased for women after pregnancy, the overall risk of breast cancer is still low in this younger age group.
"In this age group, breast cancer is uncommon," said University of North Carolina Lineberger Comprehensive Cancer Center's Hazel B. Nichols, Ph.D. "The risk of developing breast cancer is still low overall, even if you've had a child five years ago."
The team also added that the new research backs up previous studies which have also found an increase in breast cancer risk in younger women after giving birth, however these studies did not take into account other potentially influencing factors included in the new analysis.